The present invention relates to an electric power steering apparatus that assists a vehicle steering operation.
When a vehicle is traveling, a steering operation is needed in some cases even if the vehicle is in the straight-line traveling state. For example, if the vehicle is deflected by travelling on a canted road or due to the influence of a crosswind or insufficient wheel alignment, a steering operation is needed to suppress the deflection. If such deflection of the vehicle continues for an extended period, the driver needs to keep applying steering torque to the steering mechanism of the vehicle even if the vehicle is in the straight-line traveling state. The resultant continuous physical burden accumulates as fatigue of the driver, which is not favorable. In this regard, Japanese Laid-Open Patent Publication No. 2010-254178 discloses an electric power steering apparatus that executes what is referred to as lead pull compensation control, in which when steering torque is applied to the steering mechanism of the vehicle even if the vehicle is in the straight-line traveling state, an assist command value is modified to decrease the absolute value of the steering torque.